BK Channel α and γ1 Subunits Assemble with a Stoichiometry of up to 4:4, but One γ1 is Sufficient to Produce the Full γ1-Induced Gating Shift

Abstract

Ca2+-and-voltage-gated K+-channels of large conductance (BK) are symmetrical complexes whose minimal functional unit is a tetramer composed of four identical α-subunits. BK α-subunits can coassemble with auxiliary subunits belonging to two different families, β and γ, which drastically influence channel gating. The presence of β2 subunits produces a complete inactivation of BK currents and the inactivation rate at single channel level is a functional reporter of the number of β2 subunits/channel. This methodology has established that up to four β-subunits can assemble with the α-subunits in a BK channel complex, where each one contributes an identical additive increment to the total β-induced gating shift. However, the stoichiometry of the assembly of α and γ subunits is still unknown. In order to solve it, we designed an “inactivating” γ1 subunit (chimera β2/γ1) and measured the inactivation rates at the single channel level. Our results show that, similar to β subunits, up to four β2/γ1 chimeric subunits can assemble in BK channels expressed in Xenopus oocytes. Unlike β subunits, the presence of one β2/γ1 chimeric subunit in a single BK channel is sufficient to produce the full γ1-induced gating shift. Our results confirm the ‘all-or-none’ type of functional regulation produced by γ1 on BK channels. Independent FRET-based methods also confirm that up to four tagged γ1 subunits can be present per BK channel (Supported by GM118114 to CJL).